Rectifiers are generally used for supplying direct current systems from three-phase current systems, such as in the public three-phase system. These rectifiers are mostly configured as a bridge circuit, diodes being used as rectifier elements. The diodes do not need any additional activating circuit, since they transition automatically at the correct point in time into the conducting or blocking state.
Bridge rectifiers are also used as rectifiers in three-phase generators for motor vehicles. The power loss implemented in the diode rectifiers results from the diode configuration and the current to be rectified. These losses may be reduced only insignificantly by circuitry-related measures, such as parallel switching of diodes. However, if the diodes are replaced by active switches, e.g., MOSFETs, these losses may be considerably reduced. The use of active switches, however, requires a controller which switches the switches on and off at the correct point in time.
A critical operating state of an active rectifier is the load dump. A load dump is present if the load cable drops or consumers are abruptly turned off in the case of an excited machine having output current. The generator usually continues to deliver energy, which must be converted in the rectifier, for 300 ms to 500 ms in order to protect the vehicle electrical system against damage due to overvoltage.
In conventional diode rectifiers, this energy loss may be converted into heat. Here, the diodes offer an adequate integrated circuit packaging having low thermal impedances. Furthermore, the Zener diodes offer the advantage that the Zener voltage rises with rising temperature. In this way, almost even distribution of the current load of the branches is achieved among the individual branches. In the thermally balanced state, the Zener voltages are initially not identical due to manufacturing deviations. Switching branches having a lower Zener voltage are subjected to more current. A self-inhibiting effect occurs, which results in an almost even distribution of the current, due to the fact that the Zener voltage rises with rising temperature.
In the case of active rectifiers, MOSFETs are usually used as power switches. In known circuits for voltage clamping, the clamp voltage is a function of the threshold voltage of the MOSFETs, which is why the decrease in the threshold voltage results in a decrease in the clamp voltage. Since the threshold voltages of MOSFETs have negative temperature coefficients, the clamp voltage of such active rectifiers decreases when the temperature increases. This results in an uneven distribution of the current resulting in an uneven distribution of the temperature, which reinforces the uneven distribution of the current and is demonstrated as a positive feedback effect.
In known circuits for voltage clamping, the clamp voltage is composed of the breakdown voltage of a Zener diode, or a chain of Zener diodes, and the threshold voltage of the MOSFET. Since the Zener diodes are thermally not coupled or coupled only insufficiently to the MOSFET, their positive temperature coefficient cannot compensate for the negative temperature coefficient of the threshold voltage of the MOSFET.
An output stage having an even distribution of the Zener voltage and a method for operating this output stage are known from the publication WO 2006/114362 A2. The above-described output stage is used for switching inductive loads and includes at least two parallel-switched individual output stages. In parallel-switched output stages, problems may occur during switch-off due to tolerances, thus resulting in the output stage being strictly limited in its range of application. To avoid this from happening, it is proposed to carry out a thermal coupling between a Zener diode and a switching transistor which are components of an individual output stage, thus causing the output stage extinction voltage to be evenly distributed.